(1) Field of the Invention
The present invention relates to a cell safety valve that releases a gas inside a cell to the outside of the cell when an internal pressure of the cell exceeds a predetermined value. The present invention also relates to a cell having the cell safety valve.
(2) Description of the Prior Art
These days, non-aqueous electrolyte cells are attracting attention as being capable of improving capacity. The non-aqueous electrolyte cell uses, as the positive electrode material, a lithium-containing composite oxide such as LiCoO2 or LiMn2O4 and uses, as the negative electrode material, a lithium-aluminum alloy and a carbon material that are capable of intercalating and deintercalating lithium ions.
When such a non-aqueous electrolyte cell is mishandled, e.g., put in fire, charged or discharged under abnormal conditions, a great amount of gas may be produced in the cell. In such a case, the gas needs to be released out of the cell quickly. In view of this, the cell is provided with a safety valve for releasing the gas generated abnormally in the cell out of it quickly. Such a safety valve is required to have functions shown as (a)–(c) below.
(a) To operate quickly when an internal cell pressure rises abnormally (at the time of cell abnormality).
(b) To release a gas generated abnormally inside the cell quickly.
(c) Not to operate when dropping impact is applied.
Conventionally, safety valves shown as (I)–(III) below have been proposed.
(I) A cell safety valve wherein a ladder-shaped break groove 51 is formed on a sealing plate 50 and is made to break first at the time of cell abnormality, as described in Japanese Unexamined Patent Application No. 2000-348700 (see FIGS. 16 and 17).
In such a configuration of the safety valve, when an internal cell pressure rises, deformation volume of the periphery of the safety valve is so small that it is difficult for the safety valve to operate when the cell has a small internal pressure, and it is also difficult to control the operating pressure of the safety valve. In addition, the breaking of the safety valve is too small in area to release a gas and therefore speed to release the gas is slow, thus leaving a problem of failing to meet the functions (a) and (b).
(II) A dome-shaped thin valve plate 54 is formed over an opening hole 53 in a sealing plate 50, and a break groove 55 is formed near the periphery of the thin valve plate 54, as described in Japanese Unexamined Patent Application No. 11-273640 (see FIGS. 18 and 19).
Generally, in a safety valve in which a sealing plate is provided with a thin plate portion on which a break groove is formed, when an internal cell pressure rises, the valve plate 54 is deformed by the pressure. This makes it possible for the safety valve to operate at a relatively low pressure. However, in a valve plate having the above-described configuration wherein the break groove 55 is formed at the periphery of the thin valve plate 54, deformation volume at the periphery of the thin valve plate 54 is small. In addition, a stress that the valve plate 54 receives is applied equally on the break groove 55, and it is therefore impossible for the safety valve to operate at a low operating pressure. As a result, cell-to-cell variations in the operating pressure of the safety valve are made large, thus leaving a problem of failing to meet the function (a).
(III) Two dome-shaped dome portions 56 are formed over an opening hole 53 in a sealing plate 50, and break grooves 55·55 to facilitate the breaking of a valve plate are formed at the periphery of the dome portions 56·56 so that the break grooves are adjacent to each other at the substantially center portion of a safety valve, as the present applicants proposed in Japanese Unexamined Patent Application No. 2001-325934 (see FIGS. 20 and 21).
In a safety valve having such a configuration as described above, since the break grooves 55·55 are disposed in the substantially center portion (a portion having large deformation volume) of the valve plate, a stress from an internal cell pressure is concentrated on the center portion. This makes it possible for the safety valve to operate at a low operating pressure and to reduce cell-to-cell variations in the operating pressure of the safety valve. However, when an excessive dropping impact is applied on the center portion of the safety valve that has large deformation volume, it is possible for the break grooves 55·55 to break and thereby cause leakage of an electrolyte solution, thus leaving a problem of failing to meet the above-described function (c).